1. Field of the Invention
The present invention relates to clock derivation systems for telephones in a large-scale land mobile radio-telephone system utilizing the public telephone transmission circuit network as one of the main media, and particularly to a clock derivation system for deriving clock information from a Manchester code or the like having a base band of twice the clock frequency.
2. Description of the Prior Art
For deriving clock information from an input signal of base band in a digital transmission, there have been proposed two types of systems; use of a full-wave rectified signal (analog operation) and use of waveform change points (differentiated waveform) (digital operation). However, these methods have a drawback. When a Manchester code of twice the information speed, binary AMI (Alternate Mark Inversion) code or split phase code is used for a base band signal, there is a possibility that in the above two methods, phase inverted clock information is detected as correct clock information. This is caused by a fact that in the method using a fullwave rectified signal, positive and negative waves are asymmetrically rectified and in the method using waveform change points, unnecessary waves are passed due to the characteristics of the following stage of the clock wave filter and amplifier.